Download f ′(c)

4.1 Maximum and Minimum
Values
4.1 Maximum and Minimum Values
Definition:
• A function f has an absolute maximum (or global
maximum) at c if f(c) ≥ f(x) for all x in D, where D
is the domain of f. The number f(c) is called
maximum value of f on D.
• Similarly, f has an absolute minimum at c if f(c) ≤
f(x) for all x in D and the number f(c) is called the
minimum value of f on D.
• The maximum and minimum values of f are called
the extreme values of f.
Local Maximum and Minimum
Definition:
• A function f has a local maximum (or
relative maximum) at c if f(c) ≥ f(x) when x
is near c.
• Similarly, f has a local minimum at c if f(c) ≤
f(x) when x is near c.
Example:
Absolute maximum
(also local maximum)
Local maximum
Local minimum
Local minimum
Absolute minimum
(also local minimum)
Extreme Value Theorem:
If f is continuous over a closed interval, then f has
absolute maximum and minimum over that interval.
Maximum &
minimum
at interior points
Maximum &
minimum
at endpoints
Maximum at
interior point,
minimum at
endpoint
Example when a continuous function doesn’t have
minimum or maximum because the interval is not closed.
4
No
Maximum
3
2
1
yx
2
D   0, 2 
-2
-1
0
1
2
No Minimum
Suppose we know that extreme values exist. How to find them?
Absolute maximum
(also local maximum)
Local maximum
Local minimum
Notice that local extremes in the interior of the function
occur where f  is zero or f  is undefined.
Fermat’s Theorem
If f has a local maximum or minimum at
c, and if f ′(c) exists, then f ′(c) = 0 .
Note: When f ′(c) = 0 , f doesn’t necessarily have a
maximum or minimum at c. (In other words, the
converse of Fermat’s Theorem is false in general).
Example when f ′(c) = 0 but f has no maximum or minimum at c .
2
yx
3
1
-2
-1
0
1
2
f0
-1
-2
(not an extreme)
Critical numbers
• Definition: A critical number of a function f is a
number c in the domain of f such that either f ′(c) = 0
or f ′(c) doesn’t exist.
• Example: Find the critical numbers of f(x) = x1/2(x-3)
Solution:
x 3
x  3  2 x 3x  3
f ( x) 
2 x
 x
2 x

2 x
Thus, the critical numbers are 0 and 1.
• If f has a local maximum or minimum at c , then c is a
critical number of f .
The closed interval method for finding
absolute maximum or minimum
To find the absolute maximum and minimum values
of a continuous function f on a closed interval
[a,b]:
1. Find the values of f at the critical numbers of f in
(a,b) .
2. Find the values of f at the endpoints of the
interval.
3. The largest of the values from Steps 1 and 2 is
the absolute maximum value; the smallest of
these values is the absolute minimum value.
Examples on the board.